I can tell a lot of classes/specs that are able to reach high rating without a single pve item, such as:
- Rshaman;
- Disc Priest;
- Hpala;
- Unh DK;
- Retri Paladin (the PvE items are an amazing boost but still, it's possible to play it at high rating and still be good);
- Feral;

And basically anything played by someone who actually knows how to play his class properly besides going for a simple oneshot.

You don't need PvE items for PvP, but they do help a lot. For example, You can get a 10% increase in damage at the cost of 5% damage reduction. But if you approach resilience mathematically, it reduces damage linearly of the time you survive but the % damage reduction is an exponential function that diminishes. Also, you can approach resilience as increasing healing (for example, 50% damage reduction will theoretically increase healing by 2). If you still don't understand why, think of a player with 150k hp and 50% damage reduction. He has 300k effective health. So healing him for 10k is equivalent to healing 20k effective health.